The most common forms of preventable and avoidable blindness globally are glaucoma, macular degeneration and diabetic retinopathy. All present with physical changes to the shapes/colours of the normal structures, the nerve and nerve/vascular layer known as the retina, at the back of the eye. Diagnosis of disease is made by direct observation of changes to the normal appearance of these three locations: the circular optic disc, the plain macula and the plain retina and vessel pattern.
Until recently, only highly skilled ophthalmologists and opticians could safely examine the back of the eye using dilating eye drops and complex medical equipment such as ophthalmoscopes and special lenses. In the last decade, major advances in imaging have led to the development of non-mydriatic cameras whereby anyone can take a digital photograph of the back of their eye, simply by placing their head on a chin rest and looking in to the camera. More recently, the ubiquitous mobile phone has camera accessories and fixtures to take the same images. Ongoing research is continuing to image the retinal layers in increasingly fine detail, suggesting the possibility of pre-damage disease detection.
Glaucoma is a condition where the optic nerve is excessively vulnerable and starts to thin out, losing ability to transmit images from the retina to the brain. Early thinning can be detected by observing the changes in the appearance of the optic disc (the head of the nerve where it leaves the eyeball), as illustrated in FIG. 1, as described below. Early detection can mean early treatment and prevention of irreversible sight loss.
In the last decade, new advances in medicine have introduced skilled procedures, such as selective laser trabeculoplasty (SLT) and micro drain implants, which can control previously uncontrolled glaucoma. The preceding decade introduced drugs called prostaglandin inhibitors which transformed the medical management of the majority of previously blinding cases of glaucoma. The fact is that in the developed world, an estimated 50% of patients with glaucoma, “the silent thief”, cannot access these new sight saving remedies and glaucoma remain undetected. Once nerve damage has occurred, vision is irretrievably lost.
Photographic examination of the optic nerve head fibres (the optic disc) as they enter the eyeball through the cribriform plate from the brain has only been accessible to specialists until recently. FIG. 1a illustrates a normal optic nerve head and blood vessels within a fundus photograph. FIG. 1b is an image of advanced glaucoma showing large pale ‘cup’ and thin neuroretinal rim (right). The large paler area (sometimes called the cup) represents the area free of axons where the nerve has been cored out′. The blood vessels branch from the centre (the central retinal artery) which can be seen to be displaced between 12 o clock and 11 o clock on the right of the rim beside the arrow, before it ‘bends’ up around the rim. In the last decade, major advances in imaging have led to the refinement of non-mydriatic cameras whereby anyone can take a digital photograph of the optic nerve head at the back of their eye simply by placing their head on a chin rest and looking at the camera. More recently, the ubiquitous mobile phone has camera accessories and fixtures using adaptive optics to take equivalent 2D images of the undilated eye. FIG. 2a is a photographic image of an optic disc from a PEEK mobile phone fundus camera attachment. FIG. 2b illustrates an example of a D-Eye phone ophthalmoscope/camera attachment.
Ongoing research is continuing to image the retinal layers in increasingly fine detail, suggesting the possibility of accurate identification, recognition and early nerve fibre disease detection, especially glaucoma. Most advanced clinical imaging of the optic nerve head uses SD-Optical Coherence tomography (OCT), a three-dimensional scanning ophthalmic camera. The latter is too complex for general use although is increasingly applicable for specialized screening.
Almost all studies heretofore have analysed the optic nerve head for glaucoma disease. Furthermore, these studies have focused on what is called the cup-disc ratio, using segmentation of the disc rim minus the inner cup, as a glaucoma index. However, a cup-disc ratio does not definitively indicate axonal optic nerve fibre loss. Furthermore, the ratio is a summary of the measurement of a specific radius of a disc, which is rarely a perfect circle. This is illustrated in FIG. 3, a schematic representation of the optic nerve head photograph images of FIG. 1. Referring to FIG. 3, AB is the center to the rim, and AC is the center to the retina. The cup/disc ratio is the proportion AB to AC. It is also well accepted amongst ophthalmologists that although an increased optic cup-disc ratio suggests a risk of glaucoma, there is a high chance of over fitting with a labeled data set from patients already diagnosed, with an unacceptable chance that glaucoma can progress with loss of axons without affecting the cup/disc ratio.
The use of imaging the retinal blood vessels as a biometric marker has been around for many years, yet it still remains a challenge to develop a safe robust biometric to address shortcomings with current biometrics, such as retinal scans, fingerprints and iris scans. Table 1 summarises relevant research on retinal biometrics. Ahmed et al applied a method using semicircular discs around the optic nerve head with only 84.2 and 89.2% accuracy. Kose et al employ vessel segmentation of similarity (length) measurements with circular sampling.
TABLE 1Summary of retinal biometric studiesJiu et al. 2016Pre-processingRetinal vesselRandomClinical data setswith featurevector analysis ofbifurcation pointsonly, small. 93%extractionbifurcation points.for circle chosen.accuracyTraditionalVector alongRetina analysedmachine learninglength of vesselKose et al 2011Retinal vesselsegmentationwith circularsampling andvessel lengthvectorDrozd 2012Retinal vesselPoor optic discRetina analysed8% best resultbifurcationlocalisationanalysedBevilacqua et alBifurcation pointsCloud of pointsRetina analysed2008on retinaAhmed et alOptic nerve headSemicircularOptic nerve head2012segmentsectionexamined
The unique image of the optic nerve head is from a fixed environment without variation in lighting conditions, which hamper pupil size for retinal scans or pupil light change for iris scans. The disc image is inaccessible without full-directed gaze and compliance from the individual, unlike the iris images, which can be captured remotely and reproduced illegally. The optic disc is approximately 1-2 mm in diameter, close to the back of the eye and with unique features making it significantly more accessible, more accurate and easier to image than full retinal blood vessels scans.
It has been suggested that there is a decrease in cup-to-disk ratio and neuroretinal rim area as age increased in studies based on Asian populations. FIG. 4 is a diagrammatic illustration of what happens to the position of the blood vessels in the optic nerve head when thinning of the neuroretinal rim occurs over time. FIG. 5 is a photographic image of the optic nerve head of a patient with progressive glaucoma over ten years, demonstrating enlargement of the central pale area (cup) as the rim thins, with displacement of their blood vessels.
In view of the above, there is a need for an improved method and system for detecting and analysing changes in the optic nerve head.